Synthesis and Characterization of Imprinted Polymers for Radioactive

3730

Ind. Eng. Chem. Res. 2009, 48, 3730–3737

APPLIED CHEMISTRY Synthesis and Characterization of Imprinted Polymers for Radioactive Waste Reduction Anupkumar Bhaskarapillai,†,§ Narasimhan V. Sevilimedu,*,‡ and Bo¨rje Sellergren*,† INFU, UniVersity of Dortmund, Otto Hahn Str-6, 44221 Dortmund, Germany, and WSCD, Bhabha Atomic Research Centre Facilities, Kalpakkam-603102, India

A cobalt(II) imprinted polymer, that could lead to significant reduction in radioactive waste volume generated during decontamination of nuclear reactors, was synthesized using [N-(4-vinylbenzyl) imino]diacetic acid as the functional monomer through two different methods. The imprinted polymer was found to selectively trap cobalt(II) ions even when present in ppb levels from strong complexing medium against excess ferrous ions. It showed no uptake of ferrous ions under the conditions tested. Cobalt sorption and desorption were found to be rapid. The cobalt selectivity of the polymer was also demonstrated with radioactive cobaltous ions wherein it showed 55% reduction in cobalt activity from a complexing solution containing 2 µCi of radioactive cobalt along with excess ferrous ions. The selectivity of the polymer was compared to that of a commercial resin containing similar functional ligand. 1. Introduction A variety of structural materials such as carbon steel, Monel, Incoloy, stainless steel, and Stellite are employed in the cooling water systems of power plants. These materials interact with the coolant at high temperature and form deposits of metal oxides. In case of nuclear power plants, the problem further aggravates because the metal oxide deposits trap radioactive metal ions in the oxide lattice leading to buildup of radioactivity. The radioactive metal ions are generated during normal operation of the nuclear reactors, as the corrosion products are transported to the core along with the coolant and get activated by the neutrons.1 Thus, frequent cleanup campaigns, otherwise known as decontamination campaigns, are necessitated. During such cleanup operation, which is generally done by continuous circulation of dilute chemical formulations containing complexants (dilute chemical decontamination), the metal ions are brought out into the solution and trapped by the ion-exchange resin beds that form a part of the cleanup circuit.2-4 The primary cooling water circuits of pressurized heavy water reactors (PHWR), for example, are made of carbon steel.3 Hence, during chemical decontaminations of these reactors, a large quantity of ferrous ions (large part of which is nonradioactive) is released into the coolant. Because there is cobalt impurity present in some of the alloys used (nickel-containing alloys and to some extent in carbon steel) and also there are some parts that are made of cobalt-containing alloys (e.g., Stellite, which has 50% cobalt), a small quantity of cobaltous ions also gets incorporated into the corrosion product oxides and gets activated to become radioactive cobalt.1 Chemical cleaning of the reactors is carried out to remove such activities. Cobalt is a major contributor toward the radiation build-up problem because of its rather long * To whom correspondence should be addressed. B.S., Tel.:+49231-7554082. Fax: +49-231-7554084. E-mail: [email protected]. N.V.S., Tel.: +91-044-27480203. Fax: +91-04427480097. E-mail: [email protected]. † University of Dortmund. ‡ Bhabha Atomic Research Centre Facilities. § Present address: WSCD, Bhabha Atomic Research Centre Facilities, Kalpakkam-603102, India.

half-life (t1/2 ) 5.27 y) and high energy (sum peak of 2.5 MeV) gamma emission. Because the normal ion-exchange resins currently used in general in reactors lack high selectivity toward cobaltous ions in the presence of ferrous ions, it leads to the generation of a large amount of radioactive ion-exchange waste, which requires costly and elaborate disposal procedures. The volume of radioactive waste can be substantially reduced, if a resin that is selective for cobalt and preferably excludes ferrous ions were available, as all of the cobalt activity could be trapped within a very small volume of the resin (Figure 1). However, literature search has not revealed any cobalt selective resin that exhibits exclusion of ferrous ions, although there are commercially available chelating resins that show cobalt selectivity over ferrous ions, for example, Amberlite-IRC 718.5,6 The objective of the present study was, therefore, to synthesize a resin that exhibits high selectivity for cobalt against ferrous ions. Molecular imprinting7-9 has been reported widely for achieving selective adsorbents for particular molecules and metal ions, and there are also few reports wherein selective adsorbents for anions10,11 were prepared by this technique. These are highly cross-linked polymers synthesized in the presence of molecular/ metal templates, which are subsequently removed to get the imprinted polymer. The general procedure for the synthesis of metal imprinted polymers (MIP) involves either a one-pot synthesis of the imprinted polymer wherein polymerizable complexant, metal salt, and the cross-linker are taken together and subjected to polymerization,12,13 or a two-step process wherein the complex is isolated and the isolated complex is used in the polymerization.14,15 Several examples of bulk and surface metal imprinted polymers have been reported, which exhibit pronounced selectivity for particular metal ions.16 There are also reports on utility of metal imprinted polymers in removal of uranium, which will have applications in uranium extraction and treatment of nuclear effluents.15,17,18 A survey of the literature, however, did not reveal any report on cobalt imprinted polymers tested against ferrous ions except for a patent.19 Yet, this shows a high capacity for ferrous ions. Very high selectivity, preferably exclusive selectivity, for cobaltous

10.1021/ie801640b CCC: $40.75  2009 American Chemical Society Published on Web 03/18/2009

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Figure 1. Schematic of a typical decontamination campaign (left). The possible scenario, if a cobalt selective sorbent such as a metal imprinted polymer (MIP) is used, is depicted on the right.

ions against excess ferrous ions in complexing medium is very essential if it were to be used in nuclear applications for reducing the radioactive waste volume. Effendiev and Kabanov20 synthesized imprinted polymers for Cu, Ni, and Co by complexation of copolymer of acrylic acid and diethylvinylphosphonate with the respective metal ion followed by solid-state cross-linking polymerization of the metal-copolymer complex with N,Nmethylene diacrylamide. These show very slow kinetics and have not been tested against ferrous ions. Our attempts to synthesize cobalt selective polymer using the same functional ligands did not yield any positive results. Moreover, cobalt imprinted polymer synthesized using the surface template polymerization technique by Tsukagoshi et al.21 did not show any uptake of cobalt ions. This contrasts with the herein reported imprinted polymers synthesized using [N-(4-vinylbenzyl)imino]diacetic acid (VbIDA) as the complexing monomer. These are capable of specific sorption of cobaltous ions in the presence of ferrous ions from strongly complexing media and have thus the potential to bring about as much as 80-90% reduction in the radioactive waste volume. 2. Experimental Section 2.1. Materials and Instrumentation. Iminodiacetic acid obtained from Sigma-Aldrich (Steinheim, Germany) and cobaltous nitrate obtained from Acros (Geel, Belgium) were used as received. Ethylene glycol dimethacrylate (EDMA) obtained from Sigma-Aldrich (Steinheim, Germany) was purified by extraction with 10% NaOH, washing with brine, drying over anhydrous magnesium sulfate and subsequent distillation under reduced pressure, and used. Azo-bis-isobutyro nitrile (AIBN) obtained from Acros (Geel, Belgium) was recrystallized from dry methanol. Amberlite IRC-718 resin was obtained from HiMedia, Mumbai, India. It was washed successively with 6 M HCl, water, 1 M NaOH, water, 1 M HCl, and water to remove the impurities before use. Dry methanol was obtained from Sigma-Aldrich (Steinheim, Germany) and stored over molecular sieves (3 Å). Ultra pure water was used for preparing metal ion solutions. Metal ion estimations at ppm level concentrations were done with a GBC 902 atomic absorption spectrometer (AAS), and ppb level concentrations were measured with ICPMS (Plasmaquad 3, VG Elemental, U.K.). Infrared spectra were recorded using KBr pellets via a Nexus FT-IR spectrometer. For radioactive experiments, radioactive cobalt source obtained from BRIT, Mumbai, India, was used. A well-type NaI-Tl scintillation detector was used for radioactivity measurements. Raman spectra were recorded with a Jobin-Yvon (France) HR800 laser Raman spectrometer using a 514 nm He-Ne laser, 1800 g/mm grating, and CCD detector. 2.2. Synthesis of [N-(4-Vinylbenzyl) imino]diacetic Acid (VbIDA). The functional ligand [N-(4-vinylbenzyl)imino]diacetic acid was synthesized with slight modification of a procedure detailed in the literature.22 A slightly modified method

was used as we could not get pure crystals through the reported procedure. Iminodiacetic acid, 3.99 g (30 mmol), and NaOH, 2.10 g (52.5 mmol), were dissolved in 1:1 v/v mixture of 60 mL of a methanol/water mixture. 4-Vinylbenzyl chloride, 4.30 g (30 mmol), was added slowly to the solution from a dropping funnel over 30 min at 60 °C. Another lot of NaOH (660 mg) was then added and was allowed to react for 45 min at 60 °C. After vacuum evaporation to one-half the volume and repeated extractions (four times) with diethyl ether, the aqueous phase was further diluted to more than twice the volume with water, and the pH of the water phase was adjusted down to 1.0 with concentrated HCl. The solution was left in the refrigerator, and pure white crystals have formed over a period of 24 h. The crystals were filtered, washed with ether, and then washed repeatedly with small amounts of water. The pure crystals were dried and used. H1 NMR (400 MHz, CD3OD) of CH2dCH-Ph-CH2-N-(-CH2COOH)2: δ 3.95 (4H, s, N-CH2-); δ 4.37 (2H, s, -CH2-); δ 5.27 (lH, d, CH2d); δ 5.82 (IH, d, CH2d); δ 6.75 (lH, m, dCH-); δ 7.45 (4H, m, -C6H4-). 2.3. Synthesis of Bis(vinylbenzyliminodiacetato)cobaltate(II) Complex. An analogous procedure as reported for copper complex23 was followed. In about 50 mL of deionized water was suspended 10 mmol (2.5 g) of VbIDA, and it was dissolved by raising the pH to 9.0 using 1 M NaOH. To this solution was added 5 mmol (1.45 g) of Co(NO3)2 · 6H2O dissolved in 150 mL of deionized water very slowly from a dropping funnel under constant stirring. Throughout the addition, the pH was maintained between 8 and 9 using 1 M NaOH. After the addition was over, the solution was filtered to remove any insoluble portion. The filtrate was freeze-dried to remove the water. It was then stirred with dry methanol for 15 min and filtered. The filtrate was evaporated to dryness. The solid obtained was again stirred with methanol, filtered, and the filtrate evaporated. This was repeated once, and the solid obtained was used as the disodium salt of the complex as attempts to get pure crystals have failed. Elemental (C, H, N) analysis showed the following values. C, 30.87; H, 3.19; N, 8.05; and sodium was found (by flame photometry) to be 13.60%. The amount of cobalt present in the complex was determined by AAS and found to be 5.66%. These values are close to the composition Co(VbIDA)2Na27H2O + 3.5NaNO3 (C26H40CoN5.5Na5.5O25.5) for which the calculated values are C, 30.53; H, 3.94; N, 7.53; Co, 5.76; and Na, 12.36%. Thus, the elemental analysis showed that the complex was isolated along with a significant amount of NaNO3 as impurity. 2.4. Synthesis of Polymers. 2.4.1. Two-Step Procedure (MIP1, NIP1). Bis(vinylbenzyliminodiacetato)cobaltate(II) (180 mg, 0.346 mmol of complexed ligand monomer), EDMA (1.25 g, 6.31 mmol), and AIBN (initiator; 15 mg, 1 wt %) were dissolved in 5 mL of dry methanol in a vial. The vial containing the solution was subjected to three freeze-thaw cycles, sealed, and kept for polymerization at 62 °C for 24 h in an oven. It

3732 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009

Figure 2. The cobalt complex, bis(vinylbenzyliminodiacetato)cobaltate(II).

was then cured at 75 °C for 24 h. The polymer formed was crushed and washed with methanol to remove any unreacted monomers. The crushed polymer was subjected to extraction of the cobaltous ions from the polymer using diluted (0.1 N) HCl until there was no more cobalt in the acid extracts. The polymer was then washed with water and methanol and vacuumdried. The polymer obtained was used as the imprinted polymer (MIP1). Nonimprinted polymer was synthesized with the free ligand (VbIDA) in place of the cobalt complex. The nonimprinted polymer was crushed and washed with methanol and with pH 8.0 sodium hydroxide solution to ensure removal of any unreacted reactants. It was then washed thoroughly with 0.1 N HCl, water, methanol, dried, and used as nonimprinted polymer (NIP1). 2.4.2. One-Step Procedure (MIP2, NIP2). Co(NO3)2 · 6H2O, 46.6 mg (0.16 mmol), VbIDA, 79.7 mg (0.32 mmol), and tetrabutyl ammonium hydroxide, 0.58 mL of 1 mM methanol solution (0.58 mmol), were taken in 5.0 mL of dry methanol and stirred at room temperature for 15 min. To this solution were added 1.20 g (6.4 mmol) of EDMA and 15 mg of AIBN (1 wt %), which was freeze-thawed thrice and then polymerized at 62 °C for 24 h followed by curing at 75 °C for another 24 h. The polymer obtained was crushed and treated in the same way as explained above to get MIP2. Nonimprinted polymer (NIP2) was synthesized the same way as the MIP2 except that no cobalt salt was taken in the reaction mixture. 2.5. Batch Adsorption Experiments. All of the batch adsorption experiments were done in triplicates, and the mean was taken. Unless otherwise noted, all of the adsorption studies were done by shaking 25 mg of the polymer with 2.5 mL of appropriately buffered metal ion solutions to equilibration in a vial for 30 min. The metal ion uptake (retention capacity) was calculated by measuring the metal ion concentrations in the initial solution and in the supernatant separated after centrifugation of the equilibrated experimental solution. Metal ion concentrations were measured using AAS or ICP-MS. 3. Results and Discussion 3.1. Polymer Synthesis. The imprinted polymers were synthesized by two methods: (1) a two-step procedure in which complexation was performed and then the isolated complex (Figure 2) was cross-linked by thermal polymerization, and (2) a one-step procedure in which complexation and polymerization were done in one pot wherein an organic base (tetrabutyl ammonium hydroxide) was used to keep the ligand in solution. The first method was pursued to ensure that pure complex is used in the imprinting process.13 Yet, despite repeated attempts,

the isolated complex could not be purified completely, as evidenced by the elemental analysis results, which indicated the presence of sodium nitrate impurity along with the complex. The complex was used as such for the synthesis of MIP. In terms of the performance of the imprinted polymers, as would be shown below, polymers synthesized through both of the methods have shown similar behavior. The presence of impurity in the complex synthesized is thus shown not to interfere in the synthesis of a performing MIP. The nonimprinted polymer synthesized by the one-step method (NIP2) showed adsorption of metal ions unlike the one synthesized by the second method in which no organic base was used in the polymerization medium. This could be attributed to the limited solubility of the free ligand. In fact, the organic base is ensuring proper incorporation of the ligating groups in the cross-linking network. Thus, a workable NIP requires the presence of base to keep the free ligands in solution during the synthesis of the NIP. Synthesis of imprinted polymers (MIPs) involved acid extraction of the cobalt ions (template) from the cross-linked polymers as the last step. The extracted solutions were analyzed by AAS for the amount of extracted cobalt. The theoretical capacity of the imprinted polymers could be calculated thus and was found to be 94.6 µmol/g for MIP1 and 108.3 µmol/g for MIP2. From the quantity of the cobaltous ions taken for the polymer synthesis, it could be inferred that more than 90% of the originally added template ion has resulted in functional binding sites. IR and Raman spectra of the free ligand and the MIPs confirmed the incorporation of the ligand groups in the imprinted polymer. 3.2. Spectroscopic Analysis. The IR spectra of the free ligand, MIP1, and MIP2 are shown in Figure 3. The IR spectrum of the free ligand showed the characteristic peaks for the carboxylic -OH at 3378 cm-1; CH stretch at 2950-3000 cm-1; carbonyl at 1726 cm-1; vinyl peaks at 1630 (CdC) and 1400 cm-1 (dCH2); C-O stretch at 1250 cm-1; and aromatic peak at 883 cm-1. The hydroxyl (>3400 cm-1) and aromatic (881 cm-1) peaks of the ligand could be seen in the spectra of both MIPs, indicating the incorporation of the ligands into the polymer. The other peaks of MIP1 and MIP2 and their assignments are:24 2850-3000 cm-1, CH stretch; 1726 cm-1, carbonyl; 1630 cm-1, unreacted vinyl CdC; 1380-1470 cm-1, -O-CH2 bending; 1150-1260 cm-1, C-O, ester; 962 cm-1, C-CH3 rocking; and 750-800 cm-1, CH vinyl out-ofplane bending. A better distinction of the frequencies of the functional ligand groups and the cross-linking monomer was possible with Raman spectra (Figure 4). The Raman spectrum of the free ligand, VbIDA, showed characteristic vinyl peaks at 1629 (vinyl (CdC)) and 1406 cm-1 (vinyl dCH2); aromatic peaks at 1610, 1185, 831, 850, and 640 cm-1; and ν (C-X) at 1208 cm-1 (also an unassigned peak at 1314 cm-1 typically seen with aromatic monomers).25 The MIP spectrum showed the characteristic aromatic frequencies of the ligand, at 1612, 1189, 817, and 882 cm-1, which again clearly indicate the incorporation of the functional ligands in the polymeric network. The strong peak at 1458 cm-1 in MIP is due to the methyl group of the cross-linking monomer, and the one at 1727 cm-1 is due to the carbonyl group of the crosslinking monomer. The very small peak at 1629 cm-1 in MIP indicates that only a very small fraction of the double bonds remains unreacted in the MIP. 3.3. Batch Adsorption Studies. 3.3.1. Cobalt Uptake from Complexing Citrate Medium. Metal ion solutions prepared in 0.1 M citrate buffer were shaken with the polymers

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Figure 3. IR spectra of (a) free ligand (VbIDA) and (b) imprinted polymer, MIP1 (two-step synthesis), and (c) imprinted polymer, MIP2 (one-pot synthesis).

Figure 4. Laser Raman spectra of (a) free ligand (VbIDA) and (b) imprinted polymer, MIP2. Table 1. Retention Capacity of the Imprinted Polymers in Citrate Medium (0.1 M, pH 4.8) sample no. 1 2 3 4 5 6 a

Figure 5. Effect of equilibration time on cobalt uptake.

(MIP or NIP) to equilibration. The effect of equilibration time on uptake of cobalt from a 250 ppm (4.24 mM) cobaltous ion solution at pH 4.8 (0.1 M citrate buffer) is shown in Figure 5. It was found that the equilibrium was achieved within 10 min. Batch rebinding studies for competitive extraction of cobaltous ions against ferrous ions indicated that both MIP1 and MIP2 behaved similarly as shown in Table 1. These were carried out at pH 4.8 in 0.1 M citrate buffer. Unlike the imprinted polymers,

capacity (µmol/g)a

metal ion (M2+)

concentration (mM)

MIP1

MIP2

Co Co Fe Fe Co Fe Co Fe

4.24 8.48 4.48 8.96 4.24 4.48 2.12 2.24

53.4 ((4.2) 57.6 ((4.4) BDL BDL 50.8 ((4.3) BDL 25.0 ((1.8) BDL

58.0 ((4.6) 63.5 ((5.1) BDL BDL 59.7 ((4.7) BDL 29.7 ((2.2) BDL

BDL: Below detectable limit.

the nonimprinted polymers NIP1 and NIP2 were found to be different. Both NIPs did not show sorption of any of these metal ions under complexing conditions. Yet NIP2 (vide infra) showed uptake of metal ions under noncomplexing conditions, although NIP1 did not show any considerable uptake under noncomplexing conditions as well. Unlike the MIP, the lack of organized functional cavities in NIP makes it an unfavorable choice for the metal ions when present in a complexing medium such as citrate buffer. Imprinted polymers were tested for the performance under four different pH values in citrate medium. The

3734 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 Table 3. Retention Capacity of MIP2 at ppb Level Concentrations of Cobalt sample metal ion metal ion NTA concentration no. (M2+) concentration (mM) (mM)

uptake (%)

0.16 × 10-3 0.89 0.16 × 10-3 0.89

57 ((3) BDL 52 ((3) BDL

1 2

Figure 6. Effect of pH (citrate buffer) on retention capacity of MIP for cobalt. Table 2. Retention Capacity of the Imprinted Polymers in the Presence of Strong Complexant NTA at pH 4.8 (0.1 M Citrate Buffer) metal ion NTA sample metal concentration concentration 2+ no. ion (M ) (mM) (mM) 1 2 3 4 5a

Co Fe Co Fe Co

4.24 4.48 4.24 4.48 8.40 × 10-2

4.0 4.0 8.0 8.0 0.1

capacity (µmol/g) MIP1

MIP2

57.2 ((4.1) BDL 37.0 ((2.1) BDL 9.5 ((1.0)

59.3 ((4.9) BDL 40.1 ((2.9) BDL 9.8 ((1.6)

a Solution volume used: 4 mL; 70% of the metal ions were removed by the MIPs.

capacity values obtained are shown in Figure 6. The maximum capacity was achieved at a pH of 4.8 among the four pH values. This is within the pH generally maintainable during typical decontamination processes. Initially, there is increase in capacity with increasing pH, but beyond 5.0 there is reduction in the capacity. The reduction in capacity with increased pH is due to the fact that there is complete ionization of citrate groups, which is present in excess in the medium, enhancing its ability to complex cobalt. From the capacity values determined (Table 1) at different concentrations at pH 4.8 (0.1 M citrate buffer), it can be seen that the capacity for cobaltous ions reaches its maximum at a concentration of 4.24 mM. 3.3.2. Cobalt Uptake from Strongly Complexing Medium. To check the ability of the polymers to pick up cobaltous ions in the presence of strong complexing agents, batch rebinding studies were carried out in the presence of a strong complexant, nitrilotriacetic acid (NTA), which is a typical complexant used in reactor cleanup campaigns in nuclear reactors.3 The test solutions contained different concentrations of NTA along with metal ions in pH 4.8, 0.1 M citrate buffer. The results (Table 2) indicate that NTA affects the uptake only when present in excess of the metal ions. When the concentration of NTA was nearly twice that of the metal ions, the MIPs were still able to remove cobaltous ions, although with a reduced capacity. Also, importantly, there is total exclusion of ferrous ions. Here, it has to be noted that for the intended application, capacity is not the main factor but ferrous ion exclusion is. Moreover, MIPs were able to remove 70% of the cobaltous ions added even at as low a concentration as 0.08 mM. Also, it is evident that MIP1 and MIP2 showed almost similar behavior. Hence, in the ppb level experiments, only MIP2 was used as it is the one that could be synthesized by a cleaner procedure than the MIP1. 3.3.3. Evaluation of MIP at ppb Level Concentrations of Cobaltous Ions. Rebinding experiments were carried out with solution containing ppb level concentration of cobalt to evaluate its performance in low concentrations. These were carried out by equilibrating 14 mL of the metal ion solution with 25 mg of

Co Fe Co Fe

0.00 1.20

the polymer. The solution, along with the metal ions, also contained 0.02 M citrate buffer of pH 4.8. The results obtained for MIP2, tabulated in Table 3, indicate that it can remove cobalt present in ppb level concentration even in the presence of strong complexant NTA. Also, there was no uptake of ferrous ions even when they are present in large excess. 3.3.4. Evaluation of MIP with Radioactive Cobalt. To simulate the applicability of the MIP under typical decontamination conditions, experiments were carried out with radioactive cobalt in the presence of a large excess of ferrous ions and a mixture of complexing agents. The experimental solution contained cobalt activity of 0.8 mCi/L and 4 mM of ferrous ions in a typical decontamination formulation containing a mixture of complexing agents, NTA (1.4 mM), ascorbic acid (1.7 mM), and citric acid (2.4 mM). 2.5 mL of this solution (total cobalt activity in 2.5 mL of solution was 2 µCi) was equilibrated with 25 mg of the imprinted polymer. The retention capacity for the active cobalt was found to be 44.0 µCi/g ((0.7), leading to about 55% reduction in the solution activity. There was no measurable uptake of ferrous ions. This indicates that the polymer can remove the cobaltous ions even from complexing solutions containing low levels of radioactive cobalt ions as to be found during decontamination of nuclear reactors. 3.3.5. Sorption of Other Transition Metal Ions. Ferrous ion is the main competitor ion present in excess during decontamination of nuclear reactors. Yet other ions, although not in excess as the ferrous ions, could also be present in the decontamination solutions of nuclear reactors, depending on the type of materials used in the reactors. For example, a significant amount of Cr(III) and small amounts of Ni(II) ions could be present in decontamination solution of reactors having stainless steel as the structural material. Retention capacities of the MIP2, determined by batch adsorption experiments, for various transition metal ions in citrate buffer at pH 4.8 and initial metal ion concentration of 4.2 mM are shown in Figure 7. It is evident that other than cobalt(II), MIP shows high capacity only for copper(II) ions with no uptake of ferrous ions and very little sorption of nickel(II) ions, whereas in the presence of strong complexant NTA, MIP shows uptake only of copper ions (Table 4). These results show that only the imprint ion (Co2+) and copper ions form complexes with the MIP, which are stable

Figure 7. Retention capacity of MIP2 for various metal ions at pH4.8 in 0.1 M citrate buffer.

Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3735 Table 4. Retention Capacity of MIP2 for Transition Metal Ions in the Presence of Strong Complexant NTA at pH 4.8 sample no.

metal ion

metal ion concentration (mM)

NTA concentration (mM)

capacity (µmol/g) MIP2

1 2 3 4a

Cu(II) Ni(II) Cr(III) Cu(II)

4.0 4.2 4.8 8.0 × 10-2

8.0 8.0 9.5 0.1

39.0 ((2.9) BDL BDL 8.2 ((0.8)

a Solution volume used: 4 mL; 70% of the metal ions were removed by the MIP.

Figure 9. Elution of sorbed cobalt from MIP2 using HCl.

Figure 8. Effect of pH (acetate buffer) on retention capacity of MIP for cobalt. Table 5. Retention Capacity of MIP and NIP in Noncomplexing Medium (0.1 M Acetate Buffer, pH 4.8) capacity (µmol/g)

metal ion (M2+)

concentration (mM)

NIP2

MIP2

MIP1

Co Cu

4.24 4.00

31.4 ((1.8) 124.1 ((7.3)

110.2 ((7.2) 153.9 ((9.5)

98.4 ((6.1) 132.3 ((8.0)

enough to compete with strong complexant such as NTA. This indicates the wide applicability of the MIP for selective cobalt removal from various types of reactors. Apart from the intended nuclear application, the fact that the MIP does not remove much of nickel ions from the citrate medium opens the possibility for its metallurgical application in separation of cobalt from nickel, which is of high importance in the field of nickel metallurgy.26 This particular applicability needs further detailed investigations under metallurgical conditions. 3.3.6. Evaluation of MIP in Noncomplexing Medium. Although the main interest of the work is a MIP that works in complexing medium, it was also tested in a noncomplexing acetate medium to know whether the theoretical capacity had been achieved. The effect of pH on cobalt sorption in acetate medium (0.1 M acetate buffer) is shown in Figure 8. Unlike in citrate medium, there is no competition from acetate ions for complexation. This results in a continuous increase in capacity values with increasing pH until attainment of a maximum capacity at pH 4.8. This is presumably the result of an increased ionization of the carboxylic groups of the functional ligand in the same interval. The capacity values for both MIP1 and MIP2 at pH 4.8 in 0.1 M acetate buffer (Table 5) were found to be close to the estimated theoretical capacity values (94.6 and 108.3 µmol/g, respectively). Thus, the theoretical capacity could be achieved during rebinding under noncomplexing conditions. In the case of the NIP (vide supra), the NIP1 did not show any uptake, but NIP2 showed a capacity of 31.0 µmol/g for cobalt in acetate medium. Rebinding was not done with ferrous ions in acetate medium because of interference from iron hydroxide precipitation at this pH. Sorption of copper, which is known to have the highest affinity for the iminodiacetate ligands, was also

tested. As was expected, due to the very high stability of copper complexes, both MIP and NIP showed very high affinity for the copper ions (Table 5). The relative selectivity coefficient, kCo/Cu,MIP/kCo/Cu,NIP, which is a measure of selectivity for cobalt against copper due to imprinting,15,27 was calculated to be 2.86. It is to be noted that this was achieved with copper, which has a very high affinity for iminodiacetate ligands. This implies that fixing of configuration plays a major role in facilitating cobalt sorption as evidenced by much higher uptake by MIP as compared to that by NIP. 3.4. Desorption and Reuse. Desorption of the adsorbed Co(II) ions was studied in batch experiments by shaking 25 mg of the cobalt loaded MIP2 with 2.5 mL of 0.1 and 0.5 M HCl solutions. Desorption was found to be rapid and complete as shown in Figure 9. The MIPs were tested for their capacity for rebinding on reuse after desorption with 0.1 M HCl. It was tested for at least five cycles of reuse, and there was no significant reduction in either rebinding capacity or the selectivity of the polymer on reuse. 3.5. Extent of Possible Reduction in the Radioactive Resin Amount. The amount of cobalt activity removed in typical decontamination campaign in a pressurized heavy water reactor (PHWR) is about 13 Ci. From section 3.3.4, it is seen that 25 mg of MIP removes 1.1 µCi from a typical decontamination formulation. Hence, the amount of MIP that would be required to remove 13 Ci of cobalt activity would be 323 kg. This is against the current scenario wherein about 3500 kg (dry weight) of Co-60 containing IX resins is generated during a typical decontamination. Hence, the resin material containing the high level radioactive waste, Co-60, could be reduced to less than 10% of the amount that is actually generated now. During a typical decontamination, it is seen that copper is also found in the decontamination solution in nearly the same levels (ppb-sub ppb) as that of cobalt. As the MIP exhibits uptake properties for copper similar to those for cobalt under complexing conditions (Table 4), a similar amount of MIP may have to be used for removing the copper ions. Still, the total MIP that would be required will be less than 20% of the currently used resin amount for removing cobalt activity. This is a significant reduction considering the fact that cobalt-60 is a high level radioactive waste. The above calculation is based on batch equilibration studies. When used in the column mode, the realized capacity of the resin could be further higher.28 Apart from Cu(II), Ni(II) ions could be present in ppm (5-12 ppm) levels in decontamination solution. Also, in stainless steel systems, Cr(III) ions could be present in significant amount (up to about 6% of total metal ions). However, as the MIP does not exhibit any uptake of Ni(II) and Cr(III) ions under strong complexing conditions (Table 4), the presence of these ions will not be of any concern. This particular aspect, as stated earlier,

3736 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009

further widens the applicability of the MIP for various types of nuclear reactors. 3.6. Sorption Mechanism. Change in pH was monitored during sorption of cobaltous ions from nonbuffered medium. A decrease in pH on sorption would indicate that the sorption is through complexation by deprotonated ligands. The extent of this pH change can be correlated with the change in metal ion concentration and thus reflects the extent of metal ion uptake by complexation with the deprotonated ligands.29 A batch experiment was run for MIP2 (112 mg) with 4.2 mM, nonbuffered, cobaltous solution (12 mL), whose initial solution pH was adjusted to 4.92. The equilibrium pH was found to be 2.78, and the polymer used was found to have removed 5.06 µmol of cobaltous ions from the solution. If the cobalt ions were removed by complexation with ligands in the ratio of 1:2, and if both ligands liberate two H+ ions each, the change in hydrogen ion concentration (∆[H+]) and cobaltous ion concentration (∆[Co2+]) should follow the relation: +

∆[H ] ) 4(∆[Co ]) 2+

Figure 10. Retention capacity of Amberlite-IRC-718 for various metal ions at pH 4.8 in 0.1 M citrate buffer. Table 6. Selectivity Coefficient Values of MIP2 and Amberlite IRC-718 for Cobalt(II) over Other Metal Ions in 0.1 M Citrate Buffer Solution at pH 4.8

(1) +

The results obtained showed that 19.77 µmol of H ions was released into the solution and 5.06 µmol of cobaltous ions was removed from the solution on equilibration. This shows a ratio of 3.91, which closely agrees with the relation given by eq 1, indicating that sorption is through complexation by deprotonated ligands with a complex stoichiometry of 1:2 (M:L). After equilibration, the pH was again increased to effect further sorption, and the change in pH along with Co sorption was noted. This was continued until there was no more change in the cobaltous ion concentration. During each step, the pH change correlated well with the change in Co concentration as in the first step (i.e., ∆[H+] ≈ 4∆[Co2+]). The total sorption of cobaltous ions through such multiple steps was found to be 117.0 µmol/g, which is close to the total capacity value of the MIP (108.3 µmol/g) when a buffer was used to maintain a constant pH value of 4.80 (section 3.3.6). This indicates that almost all of the cobaltous ions were removed by complexation by the deprotonated ligands. Although the pH dependence of the capacity values shown in Figure 8 also supports this mechanism, the above experiment gives quantitative evidence herefore. 3.7. Comparison with Commercial Resins. For a comparison, a commercially available resin, Amberlite-IRC-718, which has the same chelating functional group (iminodiacetate)30,31 as the MIP, was subjected to rebinding analysis in citrate medium. Considering the very high capacity of the resin, batch experiments with this resin were done by shaking 5 mL of the metal ion solutions with 12.5 mg of the resin. From a 0.1 M, pH 4.8 citrate buffer solution containing 4.2 mM of Co(II) or Fe(II), Amberlite showed a capacity of 485 µmol/g for cobalt(II) and 125 µmol/g for iron(II). As is evident, this resin has very high capacity and also a reasonable selectivity toward cobaltous ions, but the magnitude of the selectivity is not adequate to address the present problem of separation of very low levels of cobaltous ions present in complexing medium from excess ferrous ions. The imprinted polymer, on the other hand, is very specific toward cobaltous ions against ferrous ions in complexing medium, which is very essential for reduction in radioactive waste volume. The capacity values of the Amberlite resin for various metal ions at pH 4.8 in citrate medium at initial concentration of 4.2 mM are shown in Figure 10. It is evident that cobaltous ions are not the most preferred for sorption, but Cu(II) and Ni(II) are. The selectivity coefficient values for Co2+ over other ions, S(Co2+/Mn+), which is a ratio of the respective distribution coefficients of the metal ions,18 for the Amberlite

a

Mn+

S(Co2+/Mn+), MIP2

S(Co2+/Mn+), IRC-718

Cr(III) Fe(II)a Cu(II) Ni(II)

4.82

1.36 4.11 0.18 0.82

1.01 6.70

MIP2 did not show any uptake of Fe(II).

resin and the imprinted polymer are given in Table 6. The selectivity coefficient values in the table show that the MIP exhibits a higher selectivity toward cobaltous ions than does the other resin. 4. Conclusions It is clear from the results that the polymer can totally exclude any uptake of ferrous ions and remove cobaltous ions from the complexing medium. This is the first report of a polymeric adsorbent capable of specific removal of cobaltous ions in the presence of large excess of ferrous ions. This polymer was synthesized and evaluated predominantly with an objective of solving the problem associated with the dilute chemical decontamination in PHWRs. Nevertheless, most of the nuclear reactor types used worldwide use iron-based alloys as structural materials and face the problem of radioactive cobalt being present along with excess iron.32 Moreover, its selectivity for cobaltous ions over other transition metal ions widens its applicability for reactors using different structural materials. This adsorbent is promising in effecting large reductions in the volume of radioactive waste generated during the regular cleanup operations as well as during decommissioning of these nuclear reactors. To evaluate this possibility, our present efforts are directed toward the synthesis of adsorbents in bead format in large scale. Acknowledgment We thank Dr. T. Kumar and M. Kutty from the Kalpakkam Reprocessing Plant, BARC Facilities, Kalpakkam, India, for help with radioactive work, K. Chandrasekhar, Centre for Characterization of Composite Materials, Hyderabad, India, for ICPMS analysis, and P. Chandramohan, WSCD, BARCF, Kalpakkam, India, for recording laser Raman spectra. A.B. thanks the German Academic Exchange Service (DAAD) for the visiting fellowship at the INFU laboratories. Literature Cited (1) Lister, D. H. Activity Transport and Corrosion Processes in PWRs. Nucl. Energy (J. Br. Nucl.) 1993, 32, 103.

Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3737 (2) Bradbury, D. Review of Decontamination Technology Development. Water Chemistry of Nuclear Reactor Systems 8; Proceedings of the conference organized by the British Nuclear Energy Society; Thomas Telford: London, 2000; pp 173-178. (3) Rufus, A. L.; Sathyaseelan, V. S.; Velmurugan, S.; Narasimhan, S. V. NTA-Based Formulation for the Chemical Decontamination of Nuclear Power Plants. Nucl. Energy (J. Br. Nucl.) 2004, 43, 47. (4) Rufus, A. L.; Velmurugan, S.; Padma, S. K.; Sathyaseelan, V. S.; Narasimhan, S. V.; Mathur, P. K. Ion-Exchange Considerations in Dilute Chemical Decontamination Processes Operated in the Regenerative Mode. Nucl. Technol. 1998, 122, 228. (5) Kennedy, D. C.; Becker, A. P.; Worcester, A. A. Nickel and Cobalt Recovery from Lead Smelter Residues. In Encyclopedia of Chemical Processing and Design; McKetta, J. J., Jr., Cunningham, W. A., Eds.; CRC Press: New York, 1989; Vol. 31, pp 87-106. (6) Lee, W. J. Prefeasibility Process Flowsheets for Cobalt Recovery. http://www.biomet.com.au/ Extract/CoFS.htm (updated June 1999). (7) Wulff, G. Molecular Imprinting in Cross-Linked Materials with the Aid of Molecular Templates-A way towards Artificial Antibodies. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812. (8) Sellergren, B. Imprinted Polymers With Memory for Small Molecules, Proteins or Crystals. Angew. Chem., Int. Ed. 2000, 39, 1031. (9) Molecularly Imprinted Polymers: Man Made Mimics of Antibodies and their Applications in Analytical Chemistry; Sellergren, B., Ed.; Elsevier: Amsterdam, 2001. (10) Say, R.; Erso¨z, A.; Turk, H.; Denizli, A. Selective Separation and Preconcentration of Cyanide by a Column Packed with Cyanide-Imprinted Polymeric Microbeads. Sep. Purif. Technol. 2004, 40, 9. (11) Fujiwara, I.; Maeda, M.; Takagi, M. Preparation of FerrocyanideImprinted Pyridine-Carrying Microspheres by Surface Imprinting Polymerization. Anal. Sci. 2003, 19, 617. (12) Vigneau, O.; Pinel, C.; Lemairea, M. Ionic Imprinted Resins Based on EDTA and DTPA Derivatives for Lanthanides(III) Separation. Anal. Chim. Acta 2001, 43, 575. (13) Vigneau, O.; Pinel, C.; Lemairea, M. Separation of Lanthanides by Ion Chromatography with Imprinted Polymers. Chem. Lett. 2003, 32, 530. (14) Kuchen, W.; Schram, J. Metal-Ion-Selective Exchange Resins by Matrix Imprint with Methacrylates. Angew. Chem., Int. Ed. Engl. 1988, 27, 1695. (15) Bae, S. Y.; Southard, G. L.; Murray, G. M. Molecularly Imprinted Ion Exchange Resin for Purification, Preconcentration and Determination of UO22+ by Spectrophotometry and Plasma Spectrometry. Anal. Chim. Acta 1999, 397, 173. (16) Prasada Rao, T.; Kala, R.; Daniel, S. Metal Ion-Imprinted PolymersNovel Materials for Selective Recognition of Inorganics. Anal. Chim. Acta 2006, 578, 105. (17) Saunders, G. D.; Foxon, S. P.; Walton, P. H.; Joyceb, M. J.; Port, S. N. A Selective Uranium Extraction Agent Prepared by Polymer Imprinting. Chem. Commun. 2000, 273. (18) Preetha, C. R.; Gladis, J. M.; Rao, T. P.; Venkateswaran, G. Removal of Toxic Uranium from Synthetic Nuclear Power Reactor Effluents Using Uranyl Ion Imprinted Polymer Particles. EnViron. Sci. Technol. 2006, 40, 3070.

(19) Kulkarni, M. G.; Karmalkar, R. N. Cobalt Imprinted Polymer Composition for Selective Removal of Cobalt, Process for Preparation Thereof, and Process for Removal of Cobalt. U.S. Patent 7001963, 2006. (20) Effendiev, A. A.; Kabanov, V. A. Selective Polymer Complexons Prearranged for Metal Ions Sorption. Pure Appl. Chem. 1982, 54, 2077. (21) Tsukagoshi, K.; Yu, K. Y.; Maeda, M.; Takagi, M. Metal IonSelective Adsorbent Prepared by Surface-Imprinting Polymerization. Bull. Chem. Soc. Jpn. 1993, 66, 114. (22) Morris, L. R.; Mock, R. A.; Marshall, C. A.; Howe, J. H. Synthesis of Some Amino Acid Derivatives of Styrene. J. Am. Chem. Soc. 1959, 81, 377. (23) Vidyasankar, S.; Ru, M.; Arnold, F. H. Molecularly Imprinted Ligand-Exchange Adsorbents for the Chiral Seperation of Underivatized Aminoacids. J. Chromatogr., A 1997, 77, 551. (24) Sellergren, B.; Hall, A. J. Fundamental Aspects on the Synthesis and Characterestics of Imprinted Network Polymers. In Molecularly Imprinted Polymers: Man Made Mimics of Antibodies and Their Applications in Analytical Chemistry; Sellergren, B., Ed.; Elsevier: Amsterdam, 2001; pp 21-57. (25) Noda, L. K.; Sala, O. A Resonance Raman Investigation on the Interaction of Styrene and 4-Methyl Styrene Oligomers on Sulphated Titanium Oxide. Spectrochim. Acta, Part A 1999, 56, 145. (26) Deepatana, A.; Valix, M. Comparative Adsorption Isotherms and Modelling of Nickel and Cobalt Citrate Complexes onto Chelating Resins. Desalination 2008, 218, 334. (27) Erso¨z, A.; Say, R.; Denizli, A. Ni(II) Ion-Imprinted Solid-Phase Extraction and Reconcentration in Aqueous Solutions by Packed-Bed Columns. Anal. Chim. Acta 2004, 502, 91. (28) Velmurugan, S.; Sathyaseelan, V. S.; Padmakumari, T. V.; Narasimhan, S. V.; Mathur, P. K. Behaviour of Ion Exchange Resins and Corrosion Inhibitors in Dilute Chemical Decontamination. J. Nucl. Sci. Technol. 1991, 28, 517. (29) Snukiskis, J.; Gefeniene, A.; Kauspediene, D. Cosorption of Metal (Zn, Pb, Ni) Cations and Nonionic Surfactant (Alkylmonoethers) in Polyacrylic Acid Functionalized Cation-Exchanger. React. Funct. Polym. 2000, 46, 109. (30) Lin, L.-C.; Juang, R.-S. Ion-Exchange Equilibria of Cu(II) and Zn(II) from Aqueous Solutions with Chelex-100 and Amberlite IRC 748 Resins. Chem. Eng. J. 2005, 112, 211. (31) Mendes, F. D.; Martins, A. H. Selective Sorption of Nickel and Cobalt from Sulphate Solutions using Chelating Resins. Int. J. Miner. Process. 2004, 74, 359. (32) Venkateswaran, G. Futuristic Programmes in Reactor Water Chemistry. In Emerging Trends in Desalination, Reactor Water Chemistry and Back-End Technology for Nuclear Fuel Cycle; Venugopalan, V. P., Narasimhan, S. V., Eds.; Proceedings of the Bhabha Atomic Research Centre Golden Jubilee Year Symposium; BARC: Kalpakkam, India, 2007; pp 4354.

ReceiVed for reView April 30, 2008 ReVised manuscript receiVed January 30, 2009 Accepted February 12, 2009 IE801640B